Measurement of particulate matter deliquescence relative humidity

ABSTRACT

A method includes measuring a leakage current through a particulate matter sample in a humidity chamber, the leakage current is measured as a relative humidity in the humidity chamber is incrementally increased, and plotting a logarithm of the measured leakage current as a function of the relative humidity. The plot of the logarithm of the measured leakage current as a function of the relative humidity has an inversion region, a low relative humidity region, and a high relative humidity region.

BACKGROUND

The present invention generally relates to the field of particulatematter contamination in electronic packaging for computer systems, andmore particularly to measuring the deliquescence relative humidity ofdust.

The physical environment surrounding a printed circuit board (PCB) isdefined by the temperature, humidity and gaseous and particulatecontamination in the air. Environmental factors may cause PCBs to failin two ways: First, electrical open circuits may result from corrosion,such as the corrosion of silver terminations in surface mountcomponents. Second, electrical short circuits may be caused by (a)copper creep corrosion, (b) electrochemical reactions such as ionmigration and cathodic-anodic filamentation or (c) settled, hygroscopicparticulate matter contamination reducing the surface insulationresistance between closely spaced features on PCBs. In 2006, theEuropean Union's Restriction of Hazardous Substances (RoHS) directivebanning the use of lead in solders led to changes in PCB finishes andthe elimination of lead from solders. These changes dramaticallyincreased the PCB failure rates due to creep corrosion. Another commonfailure mode during this period was that of surface mount resistorssuffering open circuits due to the corrosion of their silverterminations. Information technology (IT) equipment manufacturers havesince learned to make their hardware robust against these two failuremodes, which used to occur predominantly in geographies with high levelsof sulfur-bearing gaseous contamination. The failure mode that is muchmore difficult to deal with and eliminate is that of the electricalshort circuiting caused by the accumulated particulate matter in humidenvironments. The difficulty arises from the intermittent electricalnature of these particles and that the failure leaves no visibleevidence besides the presence of deposited particulate matter.

The rapid expansion of the IT equipment market in the pollutedgeographies of Asia that have high levels of fine particulate matter inthe ambient air and the increasing use of free cooling is introducingthis new, often intermittent, short-circuit failure mode due toparticulate matter. The source of particulate matter is both natural andanthropogenic. In terms of size, particulate matter may be divided intotwo categories: fine and coarse particles. Fine particles (<2.5 μm),such as those found in motor vehicle exhaust, diesel particulate matter(DPM), smoke and haze, are of two types: primary and secondary. Theprimary fine particles are directly emitted from a source, such as aforest fire, volcanoes, construction sites, unpaved roads, fields orsmokestacks. The secondary fine particles, which make up most of thefine particulate pollution, are those formed as a result ofphotochemical reactions in the atmosphere. This is generally due to thepresence of oxides of nitrogen and sulfur emitted from power plants,industries and automobiles. Sulfur dioxide and nitrogen dioxide interactwith <0.1 μm size carbonaceous material seed particles in a complex,multistep photochemical process to produce sulfuric and nitric acids.These acids are neutralized by ammonia from fertilizers, decay ofbiological materials and other sources to produce fine particlesdominated by ammonium sulfate, ammonium hydrogen sulfate and ammoniumnitrate. The majority of these secondary fine particles would beconsidered anthropogenic. Coarse particles, which are in the 2.5-15 μmsize range, include sea salt, natural and artificial fibers, plantpollens, and wind-blown dust. Their sources include erosion of soil andminerals and flaking of biological materials.

SUMMARY

According to an embodiment of the present disclosure, a method mayinclude measuring a leakage current through a particulate matter samplein a humidity chamber, the leakage current is measured as a relativehumidity in the humidity chamber is incrementally increased, andplotting a logarithm of the measured leakage current as a function ofthe relative humidity, the plot of the logarithm of the measured leakagecurrent as a function of the relative humidity has an inversion region,a low relative humidity region, and a high relative humidity region.

According to another embodiment of the present disclosure, a method mayinclude placing an interdigitated comb coupon including a plurality ofinterdigitated combs in a data center for a period of time sufficientfor particulate matter to settle on the interdigitated comb coupon,placing the interdigitated comb coupon with the particulate matterwithin a humidity chamber, measuring a leakage current through theparticulate matter bridging the plurality of interdigitated combs, theleakage current is measured as a relative humidity in the humiditychamber is incrementally increased, and plotting a logarithm of themeasured leakage current as a function of the relative humidity, theplot of the logarithm of the measured leakage current as a function ofthe relative humidity has an inversion region, a low relative humidityregion, and a high relative humidity region.

According to another embodiment of the present disclosure, a method mayinclude providing a printed circuit board contaminated with particulatematter, washing the contaminated printed circuit board with a deionizedwater and isopropyl alcohol solution, collecting the deionized water andisopropyl alcohol solution containing the particulate matter,concentrating the deionized water and isopropyl alcohol solution byevaporation, dispensing the concentrated deionized water and isopropylalcohol solution on an interdigitated comb coupon having a plurality ofinterdigitated combs, allowing the concentrated deionized water andisopropyl alcohol solution on the interdigitated comb coupon to dry,placing the interdigitated comb coupon including the particulate matterwithin a humidity chamber, measuring a leakage current through theparticulate matter bridging the plurality of interdigitated combs, theleakage current is measured as a relative humidity in the humiditychamber is incrementally increased, and plotting a logarithm of themeasured leakage current as a function of the relative humidity, theplot of the logarithm of the measured leakage current as a function ofthe relative humidity has an inversion region, a low relative humidityregion, and a high relative humidity region.

BRIEF DESCRIPTION OF THE DRAWINGS

The following detailed description, given by way of example and notintended to limit the invention solely thereto, will best be appreciatedin conjunction with the accompanying drawings, in which:

FIG. 1 depicts a top view of an interdigitated comb pattern coupon,according to an embodiment of the present disclosure;

FIG. 2 depicts plots of leakage current vs. percent relative humidityfor different salts on silver-plated interdigitated comb pattern at 25°C. using a linear scale, according to an embodiment of the presentdisclosure;

FIG. 3 depicts plots of leakage current vs. percent relative humidityfor different salts on silver-plated interdigitated comb pattern at 25°C. using a logarithmic scale, according to an embodiment of the presentdisclosure;

FIG. 4 depicts an exemplary plot of leakage current versus percentrelative humidity at 30° C. for particulate matter that caused powersupplies to fail in a data center location, according to an embodimentof the present disclosure; and

FIG. 5 depicts a flow chart describing the steps of a method formeasuring deliquescence relative humidity of particulate matter,according to an embodiment of the present disclosure.

The drawings are not necessarily to scale. The drawings are merelyschematic representations, not intended to portray specific parametersof the invention. The drawings are intended to depict only typicalembodiments of the invention. In the drawings, like numbering representslike elements.

DETAILED DESCRIPTION

Detailed embodiments of the claimed structures and methods are disclosedherein; however, it can be understood that the disclosed embodiments aremerely illustrative of the claimed structures and methods that may beembodied in various forms. This invention may, however, be embodied inmany different forms and should not be construed as limited to theexemplary embodiments set forth herein. Rather, these exemplaryembodiments are provided so that this disclosure will be thorough andcomplete and will fully convey the scope of this invention to thoseskilled in the art. In the description, details of well-known featuresand techniques may be omitted to avoid unnecessarily obscuring thepresented embodiments.

Embodiments of the present invention generally relate to the field ofparticulate matter contamination in electronic packaging for computersystems, and more particularly to measuring the deliquescence relativehumidity of dust.

Particle contamination that has accumulated on printed circuit boards(PCBs) may cause electrical short-circuit failures mainly becauseparticulate matter's electrical resistance decreases sharply when therelative humidity of the surrounding air increases above thedeliquescence relative humidity (DRH) of the particulate matter. Thecorrosion behavior of particulate matter is best characterized by itsDRH, which is the relative humidity of the air at which the particulatematter absorbs enough moisture from the air to dissolve in the absorbedwater and form an ionically conductive solution.

By developing a method to determine the deliquescence relative humidityof particulate matter suitable for failure analysis of electronichardware, embodiments of the present disclosure may, among otherpotential benefits, provide a method to test 1 mg or less of availableparticulate matter, so as to help prevent particulate contamination fromentering the data center and settling on information technology (IT)equipment, and provide a way to clean contaminated equipment. As such,the present embodiments may reduce failures associated with particulatematter contamination in IT equipment.

Referring to FIG. 1, an exemplary interdigitated comb pattern coupon 100(also referred to as interdigitated comb coupon) is shown, according toan embodiment of the present disclosure. In this embodiment, theinterdigitated comb coupon 100 may include industry standard test boardsIPC-B-24 Rev A. The interdigitated comb coupon 100 may include aplurality of interdigitated combs 102 (hereinafter interdigitated combs)separated by a gap (not shown) of approximately 0.5 mm which theparticulate matter has to bridge to conduct current between theinterdigitated combs 102.

The interdigitated comb coupon 100 (test board) may typically includefour independent comb pattern areas (also referred to as comb patterns)104 as illustrated in FIG. 1, allowing four particulate matter specimensto be tested simultaneously. In embodiments in which the amount ofparticulate matter available for analysis is limited to less than amilligram, the available analysis options may be limited to electricaland gravimetric methods. Since IT equipment reliability may be directlyinfluenced by changes in the electrical conductivity of the wettedparticulate matter, embodiments of the present disclosure focus ondeveloping an electrical method for measuring DRH of particulate matter(e.g. dust). It should be noted that gravimetric tests were alsoconducted to interpret and support the electrical conductivity testresults.

The electrical conductivity test method of measuring the DRH ofparticulate matter may start by dispensing the particulate matter undertest on the interdigitated comb coupon 100. Then, a voltage may beapplied across the interdigitated combs 102, and the leakage currentthrough the particulate matter bridging the interdigitated combs 102 maybe measured and plotted as a function of relative humidity. The DRH maybe obtained from the plot of leakage current versus relative humidity aswill be described in detail below.

In order to perform the described measurements, a sample of theparticular matter (dust) may be collected and dispensed on the combspattern areas 104 of the interdigitated comb coupon 100. In oneembodiment, a soft brush may be used to sweep the dust into a cleanplastic bag to transport to an analysis laboratory. It should be notedthat, in this embodiment, it may be difficult to sweep the fine dust offthe surface into the plastic bag and that once the dust is in the bag,it may be difficult to sprinkle the dust on to the combs pattern areas104 in a controlled manner.

In another embodiment, one or more witness interdigitated comb testboards, such as the interdigitated comb coupon 100, may be placed in thedata center under study for a period of time (e.g., one month), to allowsufficient time for dust particles to settle on the test board. Then,the contaminated test boards may be returned to a laboratory for DRHmeasurement.

In yet another embodiment, a contaminated PCB may be shipped to alaboratory where it may be washed with a deionized water and isopropylalcohol solution. The wash liquid containing the particulate matter maybe collected and concentrated by evaporation. Then concentrated drops,preferably 10 drops, each 10 μl in volume, may be dispensed on the combspattern areas 104 and dried. It should be noted that this may be themost convenient method of collecting particulate matter accumulated onPCBs.

In the laboratory, the interdigitated comb coupon 100 with a uniformcovering of particulate matter obtained by any of the methods previouslydescribed, may be placed in a precision humidity chamber at a constanttemperature of interest, generally in the 25-30° C. range. The startingrelative humidity in the chamber should be at the lower limit of thechamber capability. The relative humidity is raised in 10% steps and theleakage current through the particulate matter, at a predetermined setvoltage, measured and plotted versus relative humidity as will bedescribed in detail below. The deliquescence relative humidity haspreviously been derived from these leakage current vs. relative humidityplots as the relative humidity at which there is a sudden rise in theelectrical conductivity of the particulate matter.

As may be known to those skilled in the art, it is a standard practiceto measure conductivity of the particulate matter by applying a constantvoltage, typically 10 Vdc, across the interdigitated combs and measuringthe leakage current through the particulate matter. However,experimental results have shown that each time the relative humiditystepped up, there was a sudden rise in leakage current followed by adecay of the current. Also, relative humidity stepping up fromapproximately 75% to approximately 83% caused a drop in the leakagecurrent instead of the expected rise. These observations may beexplained on the basis that continuous application of 10 Vdc across thecombs may cause electrochemical reactions in the inter-comb space thatmay deplete the mobile ions and cause enough gas evolution to form vaporwhich is not electrically conductive.

Referring now to FIG. 2, leakage current vs. percent (%) relativehumidity (RH) plots of different salts on silver-plated interdigitatedcomb pattern at 25° C. using a linear scale are shown, according to anembodiment of the present disclosure. The procedure to determine the DRHof the collected particulate matter samples may start by using apotentiostat to measure the leakage current through the particulatematter bridging the interdigitated combs 102 (FIG. 1) by applying asquare waveform voltage across the interdigitated combs 102 (FIG. 1).

In this embodiment, the potentiostat was capable of measuring current inthe low nano-ampere range with substantial accuracy. The applied voltagewas limited to two cycles lasting a total of 4 seconds to minimallydisturb the electrochemistry of the particulate matter being tested. Asa first step in the test development, three salts including magnesiumnitrate [Mg(NO₃)₂], ammonium nitrate [NH₄NO₃] and sodium chloride[NaCl], with published DRH values at 25° C. of 53%, 62% and 75%,respectively, were studied. Ten drops, each 10 μl in volume, of a 0.1 wt% salt solution were dispensed on each comb pattern and dried. The combcoupons, including the interdigitated comb coupon 100, were placed in atemperature-humidity chamber and the humidity set at the lowest valuethe chamber could achieve, which may be approximately 30%.

Next, the relative humidity was raised in steps of roughly 10% and thetime at each percent relative humidity (% RH) step was approximately onehour. Leakage current was measured at the end of each % RH step andplotted versus percent relative humidity for each salt as shown in FIGS.2-3.

It should be noted that when plotting the leakage current versus percentrelative humidity using a liner scale as shown in FIG. 2, interpretingthe linear plots in terms of an intercept 200 between a low humidityasymptote 204 and a high humidity asymptote 202 being the deliquescentrelative humidity of the salts may not be accurate. More specifically,the intercept 200 does not coincide with the published values of the DRHof the studied salts (Mg(NO₃)₂, NH₄NO₃ and NaCl), since they may dependon the magnitude of the vertical scale. Therefore, in order to determinethe DRH values that may agree with the published values for Mg(NO₃)₂,NH₄NO₃ and NaCl the logarithm of the leakage currents for the threesalts are plotted versus percent relative humidity as illustrated inFIG. 3 below.

Referring now to FIG. 3, leakage current vs. percent relative humidityplots of different salts on silver-plated interdigitated comb pattern at25° C. using a logarithmic scale for the leakage current are shown,according to an embodiment of the present disclosure. As may beobserved, the curves in FIG. 3 exhibit an S shape that may be madepiece-wise linear by drawing straight lines covering an inversionregion, a low relative humidity region and a high relative humidityregion of the leakage current vs. percent relative humidity plot.

A high humidity asymptote 310 in the high humidity region of the curveintersects an inversion line 306 in the inversion region, and theintercept occurs at values very close to the published DRH values forthe studied salts as may be observed. The physical origin of the S shapeof the log (current leakage) versus percent relative humidity may beexplained as follows: When the salt is in equilibrium with humidity inthe low humidity range, a small increase in relative humidity does notchange the salt's electrical conductivity significantly because the saltstays relatively dry. When the salt absorbs enough moisture to startapproaching the deliquescence state, there is a rapid rise inconductivity resulting in the high slope of the inversion region of thecurve. When the relative humidity rises above the DRH of the salt, thesalt is fully dissolved and any further increase in humidity has littleinfluence on the electrical conductivity of the salt solution. Anintercept 320 between the high humidity asymptote 310 and the inversionline 306 may be considered to be the logical value for the DRH of thesalt because it may refer to the relative humidity where the salt hasabsorbed just enough moisture to become wet enough that any furtherabsorption of water will not increase its conductivity appreciably.

Another point of interest on the plots of FIG. 3 may be an intercept 324between the inversion line 306 and the low humidity asymptote 308, whichmay be referred to as the critical relative humidity (CRH). The CRH isthe point on the plot where the leakage current starts rising sharply asthe relative humidity is raised. It may be prudent to keep the datacenter humidity below the CRH of the particulate matter to ensure thatthe accumulated particulate matter will not deteriorate the reliabilityof the IT equipment via electrical current leakages between closelyspaced features on PCBs.

Additional experiments were conducted to study the effect of the squarewaveform voltage on the resulting DRH value derived from the leakagecurrent at 0.01, 0.1, 1.0 and 10 V.

Results have shown that voltage in the 0.01 to 10 V range may have noapparent effect on the leakage curves and the measured DRH and CRHvalues. It should be noted that this may apply to interdigitated combcoupons made of metals such as silver that do not oxidize. In theabsence of surface oxides, the technique may work at voltages as low as0.01 V. Interdigitated combs (e.g. interdigitated combs 102) made of tinplating may require higher voltages, approximately 10V, to break throughthe non-conductive tin oxide that forms readily on tin.

Similarly, the effect of metal plating on the interdigitated combs 102was also studied. The study was conducted after changes in Restrictionof Hazardous Substances (RoHS) regulations questioned the use of combsplated with other metals such as tin. Lead-free solders containing morethan 95% tin currently form the majority of the metallization on PCBs.Experimental results were in agreement with published DRH values for thestudied salts. However, if tin-plated comb patterns are used, themeasurement voltage may need to be higher (˜10 V) to overcome the effectof oxide cover on the tin plating.

To further test the validity of the proposed electrical method formeasuring DRH of particulate matter, water uptake of Mg(NO₃)₂, NH₄NO₃and NaCl was investigated using a gravimetric method. Results showed anexcellent agreement between the gravimetric water uptake and theelectrical leakage current measurements described above.

Referring now to FIG. 4, an exemplary plot 400 of leakage current versuspercent relative humidity at 30° C. for particulate matter that causedpower supplies to fail in a data center location is shown, according toan embodiment of the present disclosure. This embodiment may described afield failure case history. The electrical method described above formeasuring DRH was applied as part of a failure analysis. A plurality offailed power supplies showed no defect when functionally tested in thelaboratory which may be typical of particulate matter related fieldfailures. The power supplies were fully functional in the laboratory,most probably, because the electrically shorting particulate matterdried and became dislodged from where it was shorting the circuitry.

Following the particulate matter collection method described above, dustsamples were collected in the laboratory. More specifically, a couple ofpower supplies were washed with a deionized water and isopropyl alcoholsolution and the wash concentrated by evaporation. Ten drops of theconcentrate, each 10 μl in volume, were dispensed on an interdigitatedcomb pattern. Then, DRH measurements were performed following thepreviously described method as shown in FIG. 4.

In this embodiment, from the leakage current versus percent relativehumidity plot 400, a first intercept 420 between a high humidityasymptote 410 and an inversion line 406 corresponds to a DRH ofapproximately 64% for the tested particulate matter, while a secondintercept 424 between the inversion line 406 and the low humidityasymptote 408 corresponds to a CRH of approximately 52% for the sameparticulate matter sample.

It should be noted that the data center relative humidity may betypically higher than the obtained values of DRH and CRH, averaging 70%,which may explain the failure of the power supplies. Also, the low CRHand DRH of the particulate matter may be attributed to highconcentrations of salts, predominantly magnesium chloride (MgCl₂), foundin the humidifier water. The power supply failures may be eliminated byremoving these salts from the humidifier water using reverse osmosis.

Therefore, by applying the proposed method for DRH measuring, therelative humidity in the data center may be adjusted in order to preventelectrical current leakages between closely spaced features which maycause power supply failures.

Referring now to FIG. 5, a flowchart 500 describing the steps of amethod for measuring the deliquescence relative humidity of particulatematter is shown, according to an embodiment of the present disclosure.The proposed method may start at 502 by collecting a sample ofparticulate matter for analysis. The sample may be collected by any ofthe collection methods described above with reference to FIG. 1 anddispense uniformly at 504 on the interdigitated comb coupon 100 (FIG.1). In a preferred embodiment, a contaminated circuit board may beprovided and washed with a deionized water and isopropyl alcoholsolution, the deionized water and isopropyl alcohol solution containingthe particulate matter may be collected and evaporated to obtain aconcentrated deionized water and isopropyl alcohol solution which may bedispensed on the interdigitated comb coupon 100 (FIG. 1) and dried.

The interdigitated comb coupon 100 (FIG. 1) may be placed at 506 withina humidity chamber in which a voltage may be applied at 508 across theplurality of interdigitated combs 102 (FIG. 1). The humidity in thechamber may be raised in steps of approximately 10% relative humidityand the leakage current measured at each humidity step using a +/−1 Vsquare waveform. The relative humidity and a leakage current through theparticulate matter sample bridging the plurality of interdigitated combs102 (FIG. 1) in the interdigitated comb coupon 100 (FIG. 1) may bemeasured at 510. At 512 the logarithm of the measured leakage currentmay be plotted as a function of the measured relative humidity. Asdescribed above, when the logarithm of the leakage current is plottedagainst the measured relative humidity, an S shaped curve may beobtained having an inversion region, a low relative humidity region anda high relative humidity region.

At 514, from the plot of leakage current versus relative humidity (step512), an intercept between a first asymptote in the high humidity regionand an inversion line in the inversion region corresponds to thedeliquescence relative humidity for the particulate matter sample.Additionally, at 516, an intercept between a second asymptote in the lowhumidity region and the inversion line in the inversion regioncorresponds to a critical relative humidity value for the sameparticulate matter sample.

By applying the described method, the deliquescence relative humidityand the critical relative humidity of particulate matter may bedetermined. As a result, particulate matter, including samples of 1 mgor less, may be tested to determine DRH and CRH values which may be usedto adjust the data center environment such that particulatecontamination may be prevented from entering the data center andsettling on IT equipment. This may reduce failures associated withparticulate matter contamination. Additionally, the method described mayprovide a way to clean contaminated equipment and collect particulatematter samples.

The descriptions of the various embodiments of the present inventionhave been presented for purposes of illustration, but are not intendedto be exhaustive or limited to the embodiments disclosed. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope of the describedembodiments. The terminology used herein was chosen to best explain theprinciples of the embodiments, the practical application or technicalimprovement over technologies found in the marketplace, or to enableothers of ordinary skill in the art to understand the embodimentsdisclosed herein.

What is claimed is:
 1. A method comprising: providing a contaminatedprinted circuit board; washing the contaminated printed circuit boardwith a deionized water and isopropyl alcohol solution; collecting thedeionized water and isopropyl alcohol solution containing theparticulate matter; concentrating the deionized water and isopropylalcohol solution by evaporation; dispensing approximately 10 drops of 10ml each of the concentrated deionized water and isopropyl alcoholsolution on the interdigitated comb coupon to dry and form an uniformcovering of particulate matter; placing the interdigitated comb couponcomprising the uniform covering of particulate matter within a precisionhumidity chamber at a constant temperature; measuring a relativehumidity of, and a leakage current through, the particulate matterbridging the plurality of interdigitated combs, wherein an initialrelative humidity in the precision humidity chamber comprises alowermost value of a total capacity of the humidity chamber from whichthe relative humidity is raised in steps of 10% relative humidity ofapproximately one hour each, the leakage current is measured at eachhumidity step using a +/−1 V square waveform; plotting the logarithm ofthe measured leakage current as a function of the measured relativehumidity, wherein the plot of the logarithm of the measured leakagecurrent as a function of the measured relative humidity comprises an Sshaped curve having an inversion region, a low relative humidity region,and a high relative humidity region; finding a first intercept between afirst asymptote in the high humidity region and a second asymptote inthe inversion region, wherein the first intercept comprises adeliquescence relative humidity of the particulate matter; and finding asecond intercept between a third asymptote in the low humidity regionand the second asymptote in the inversion region, wherein the secondintercept comprises a critical relative humidity of the particulatematter sample.